The present invention relates to a blade intended to be exposed to a gas flow at high speed, during operation of a flow machine comprising the blade, which blade comprises a front end designed to face towards the incoming gas flow and a rear end.
By flow machine is meant primarily a gas turbine, but the invention can also be implemented in other types of flow machines, such as a steam turbine or a pump. The invention will be described in the following for the gas turbine application.
In addition, the invention relates to a section for a flow machine component, such a flow machine component, a gas turbine and an aircraft engine, all provided with at least one blade according to the invention.
A gas turbine comprises a compressor stage, a turbine stage and, for example in the case of an aircraft engine, for many types of aircraft engine, a fan duct or a fan stage. The gas turbine blade according to the invention can be arranged in and form part of at least one of these parts of the gas turbine.
The gas turbine blade forms thereby a component in a gas turbine, whereby during operation it is subjected to a fluid flowing rapidly along it from its front end towards its rear end. The gas turbine blade has normally an aerodynamic design and forms preferably a guide or a structurally-supporting element in a stator, but can also form a vane in a rotor.
Gas turbine blades, that form vanes, guides and structurally-supporting elements, that are used in gas turbines in, for example, aircraft engines and that are subjected to rapidly flowing fluid are well known and well documented in previous technology.
The manufacture of gas turbine blades can be carried out by constructing them from side walls or blade halves, or simply from a single plate that is then joined together at the front edge and, if necessary, at the back edge to create a front end and a rear end. As gas turbine blades are normally made of metal, the said joining together is preferably carried out by welding. The joint can alternatively be a glued joint or a joint created by a chemical reaction at the junction between the side walls, particularly if the gas turbine blade is made of a composite material that comprises plastic.
In operation, a flow of fluid at high speed passes the gas turbine blades in a direction from their front end to their rear end. Accordingly if, for example, the joint creates an unevenness on the surface where it is positioned, it can significantly affect the flow picture and hence the aerodynamics of the whole gas turbine blade. It is, of course, desirable to avoid an adverse effect on the aerodynamics caused by the joint, and to avoid degradation and weakening of the joint caused by the effect of the fluid on the joint.
According to previously-known technology, this problem is solved by precise control of the joining process to achieve high quality joints and by subsequent monitoring that the required quality has actually been achieved. This is, of course, labor-intensive and expensive.
In general, whether or not there is a joint, it can, in addition, be desirable to minimize the effect of the fluid on the front end of the gas turbine blades and to minimize the effect of the front end on the aerodynamics of the blade.
It is desirable to provide a blade of the type described in the introduction, where the design of the blade is such that there is a reduction in the stresses to which the front end is subjected during operation on account of the rapid flow of gas that passes the blade. It is also desirable to provide a blade that has a design that is such that it avoids any unevenness at the front end resulting in unfavorable effects on the flow of the gas that passes during operation, which unevenness, for example, can be caused by the manufacturing process.
The design of the blade should enable reliable and cost-effective industrial production of the said blades to be carried out.
In a blade according to an aspect of the present invention, the front end is provided with a concave area that is such that a stagnation point for the incoming flow of gas arises during operation at a distance in front of the outer blade surface that defines the concave area and in that the outer blade surface is thereby at least partially protected from the incoming gas flow. The shape and size of the concave area are thus such that the stagnation point for the gas flow arises in front of the concave area.
By stagnation point is meant a point at which the fluid has a low speed or is even stationary between the stagnation point and the blade. The speed is at least lower or essentially lower than the speed of the fluid that flows along the side walls of the blade. In other words, the fluid in the area of the front end, and more specifically within the concave area, will be stationary or will only flow slowly in conditions where fluid is otherwise passing rapidly. This makes it possible to position a joint in the concave area, to utilize material at the front end with a lower strength or resistance to wear than would otherwise have been necessary, to have a smaller thickness in the area or to make less stringent demands concerning the surface finish.
According to a preferred embodiment, the concave area extends from the root of the blade to the top. In this way, it is guaranteed that the required effect is achieved along the whole length of the blade.
According to an aspect of the invention, a joint, such as a welded joint, that joins together two side walls in the area of the front end, can be arranged and can extend within the concave area. If the concave area extends over the front end and the speed of the flow of fluid varies over the width of the concave area, the joint is suitably positioned where the flow can be expected to be the slowest. Normally, however, the joint is positioned in the area of a center line for the concave area, which center line extends in a direction from the root of the blade to the top. The joint is also advantageously positioned where the concave area is deepest.
As a result of an aspect of the invention, a joint that is arranged in the front end of the gas turbine blade has as small an adverse effect as possible on the aerodynamics of the blade and the effect on the joint of a fluid flowing over it rapidly is minimized and the effect on the areas weakened by this joint is minimized. According to a special embodiment, at least one duct is arranged through at least one of the wall parts that forms the front end, within the concave area, and the said duct opens out in one of the side walls at a distance from the front end. In this way, it is possible to control more precisely the flow of fluid beside the joint and also to affect the flow picture along the side walls by means of a suitable design of the ducts in question.
The duct, which is at least one in number, suitably opens out at an acute angle backwards relative to an outer surface of the side wall in which the duct is arranged, which outer surface surrounds the duct. This promotes so-called boundary-layer flow along the said side walls, and means that the air flow follows the outer surfaces of the side walls closely.
An aspect of the invention also comprises a section of a flow machine component, which section comprises a first and a second blade half, that are arranged a distance apart and are intended to form part of two adjacent blades in the component, which define between them a gas duct, with each of the blade halves comprising a front end intended to face towards an incoming gas flow during operation of a flow machine that comprises the component, and a rear end. The front end of each of the blade halves is provided with a concave area that is such that a blade in the component created by two blade halves from adjacent sections gives rise to a stagnation point for the incoming gas flow at a distance in front of an outer blade surface that defines the concave area and such that the outer blade surface is thereby at least partially protected from the incoming gas flow. By placing a plurality of such sections next to each other and joining them together, it is possible to build up a flow machine component for a flow machine in an advantageous way. Each of the sections is preferably cast separately and then joined together by welding. The welded joint is positioned within the concave area.
An aspect of the invention also comprises a flow machine component comprising a plurality of blades extending in a radial direction from an element to which they are attached, which blades are arranged a distance apart in the direction of the circumference of the component, characterized in that at least one of the blades is a blade designed according to the above.
In addition, an aspect of the invention relates to a flow machine component that comprises a plurality of sections that are joined together in the direction of the circumference and form a circular structure, characterized in that at least two of the said sections each form a section as described above.
Similarly, an aspect of the invention relates to a gas turbine that is characterized in that it comprises at least one blade designed as described above, and a gas turbine that is characterized in that it comprises at least one flow machine component as described above.
An aspect of the invention also relates to an aircraft engine that is characterized in that it comprises at least one blade designed as described above, and an aircraft engine that is characterized in that it comprises at least one flow machine component according to the invention.
Additional advantages and characteristics of aspects of the invention are apparent from the following detailed description and from other dependent claims.